Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where a gaseous or liquid reductant (most commonly a urea/water solution) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2. Although SCR is effective, the NOx-reduced emissions are only realized as long as the reductant is available for addition to the exhaust gas stream. In other words, when the supply of urea is depleted, the concentration of NOx in the exhaust emissions increases.
As the SCR process is becoming a possible strategy utilized to comply with environmental regulations, guidelines regarding the use of SCR are being contemplated. One such guideline may include requiring deactivation of a vehicle when the supply of urea runs low, and preventing operation of the vehicle thereafter until the supply has been replenished. Another such guideline may require limited utility, including the possibility that an engine without an adequate supply of urea would have a limited number of restarts While these guideline may adequately prevent excessive amounts of NOx from being expelled to the environment, they may be problematic. For example, a vehicle could become inconveniently stranded, costing the owner of the vehicle considerable time and resources.
An alternative strategy to disabling a vehicle in response to low levels of available urea may include providing a low-urea engine operating mode. One such mode is described in US Patent Publication No. 2006/0184307 (the '307 publication) by KOSAKA published on Aug. 17, 2006. The '307 publication discloses a travel assist system that searches a nearest service station based on navigation information, and predicts a quantity of aqueous urea solution consumed until the service station is reached. If a quantity of the remaining aqueous urea solution is not greater than the aqueous urea solution consumption by a predetermined value, a warning is provided to a user. Control conditions of a diesel engine enabling the vehicle to reach the service station without running short of the aqueous urea solution are then calculated, and an exhaust gas recirculation quantity is increased based on the calculation result. Thus, both the nitrogen oxides contained in the exhaust gas and a consumption of the aqueous urea solution are reduced.
Although the travel assist system of the '307 publication may suitably enable a vehicle to reach a service station without running completely out of the aqueous urea solution, it may still be problematic. Specifically, if the service station is closed, no longer in business, or does not stock a supply of urea, it may be possible for the vehicle to run out of urea and still be operational. In this situation, the vehicle would no longer be compliant with emission regulations. In addition, the system of the '307 patent does not provide a strategy for dealing with a sufficient supply of urea that is simply unavailable at a required rate, such as when the urea supply is frozen and melting slowly.
The system of the present disclosure solves one or more of the problems set forth above.